Pressure injection of semi-solid metal alloy (a finely divided solid metal phase blended in a molten metal phase) into a mould is of growing importance because of the strength and reproducibility of parts that can be achieved. In the semi-solid state material can be forced through small apertures at high rates, as a relatively low shear resistance is exhibited. As a result the technique yields near net shape production of complex, detailed shapes (even with thin walls) and advantageously provides high part consistency, and high production rates, with lower temperature dies.
This technique is relatively new and there is a limited understanding of the semi-solid process. While mould geometry and feed system designs were generally adapted from conventional die-casting nozzles, there are significant differences between the rheological properties of semi-solid metal alloys and the molten metals used in conventional die casting. Generally the temperatures are lower, and the flow properties are different.
In general, known horizontal injectors (as shown in FIG. 1) include pistons with chambers of constant cross-section to accommodate the pistons in reciprocating motion. An outlet of the chamber is generally concentric with the piston and is provided on an end face of the chamber where the semi-solid metal alloy is throttled to couple with one or more casting chambers or moulds. The concentric, throttled outlet encourages higher flow rates through the outlet and into the one or more mould cavities. Typically these injector outlets have undercut openings between a neck portion that communicates directly with the chamber, and the opening to one or more moulds. An example of such a horizontal injector is the model taught in JP2003251447 to Toshiyuki of Honda Motor Co. Ltd.
An improvement on this type of injector is taught in U.S. Pat. No. 7,341,094 to Manda (FIG. 3 thereof), which includes a piston chamber 14 that narrows abruptly to a neck portion (surrounded by heater element 42) to a final port which is produced in a 2 piece mould cavity. This design avoids an external chamber intermediate the injector cavity and the mould cavity, at the expense of limiting a shape of the moulds that can be injected (as they need to include space at the opening to hold the tip of the injector), and result in the imprinting of this tip on the surface of the moulded part, as well as irregular ears produced between the tip and the mould, that would typically have to be removed by post cast machining. It is noted that in the configurations shown in FIGS. 1,2,7 and 8, there is a single port.
Casting defects are a problem with high end applications of this method, generally in the form of inclusions (such as oxides or lubricants), porosities, surface blisters, etc. Some defects are related to outside layer drag, which is caused by a skin formed on the semi-solid metal alloy billet while it is dispensed into the injector, while it dwells in the injector and as the injector is actuated. The skin may be solidified to a higher degree than the rest of the semi-solid metal alloy billet, have different composition and may contain more lubricant, or oxides, especially at a bottom surface where the lubricant tends to pool. It is very difficult to avoid these problems as the material cools very rapidly on contact with the chamber. The skin has an important impact on the flow properties of the semi-solid metal alloy billet potentially leading to a folding, buckling or fracture of the skin which may introduce voids or space that is filled with lubricant, for example. While the semi-solid metal alloy is in motion, the outside skin can penetrate the middle layer and become trapped in the parts. In some applications this is tolerated, but in high-end production, it is desirable to avoid the inclusion of the skin.
Injection processes often use lubricants to ease sliding of the feedstock and/or a piston or bearing surface that forces the material out of the injector. When heated, lubricant may decompose or create oval or half-moon porosities in the finished products. As heat treatment is often used to increase mechanical properties of semi-solid metal alloy castings, the inclusion of lubricant can lead to lenticular porosities.
Furthermore the ejection of parts from moulds using this outlet structure may increase an amount of wasted semi-solid metal alloy, and require several moving parts such as slides (rails) and drawers. These can increase a complexity of the mould and makes them prone to mechanical breakdown, and may lead to more seams into which semi-solid metal alloy can infiltrate, leaving seams that may need to be removed by post cast processing.
The known outlet structure also extends a path between the chamber and the mould. This is undesirable because the path is relatively narrow and therefore exhibits a higher surface area to volume ratio which leads to a faster cooling rate and general heating losses, as well as pressure losses during filling and intensification stages of the die casting process.
One solution to the problem of lubricant and skin contamination used by many thixo mould manufacturers, including ALU Suisse is to remove the skin by adding an annular skimming trough (i.e. ring tank of FIG. 1) that encircles the injection chamber, surrounding the outlet. Applicant has experimented with home-made design incorporating the ring tank, in a manner very similar to that taught in U.S. Pat. No. 5,730,201 to Reollin Erich et al, from AluSuisse, which is currently in practice throughout the thixo semi-solid die casting community. Applicant has found that this it does not work satisfactorily. Applicant has concluded that a build-up of material in a dead zone between the ring tank and the outlet forms a sliding slope that encourages the skin to slide into the outlet, and finds highly turbulent flow of the material past the throttling entry channel.
It is noted that no annular skimming trough or other trap is taught, shown or suggested by U.S. Pat. No. 7,341,094 to Manda, and further that any skin formed in the chamber would invariably be mixed with the bulk of the semi-solid metal alloy in the rapid transition from the chamber to the neck, and would thus be inextricable from the remainder of the billet at the outlet. Such systems do not manage inclusions in the feed supply, and incorporate a sharp turn in the neck section.
Accordingly there is a need for an injector for semi-solid metal alloy casting that more effectively reduces the inclusion of the skin, especially a skin on a portion of the chamber where cooling is faster and lubricant is expected to pool. Additionally it is desirable to reduce a length of a channel between the piston chamber and the mould cavity, to reduce thermal and pressure losses, and simplify demoulding.